JPH0550872B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an infrared detection device using a - group compound semiconductor material.

<Prior art and its problems> There is currently a great demand for systems capable of detecting infrared radiation, especially for visualizing nighttime scenes. Infrared radiation detection systems are well known in the art, and there are a variety of different types of detection systems. Infrared radiation detection systems that have received considerable attention in recent years include those that use alloys of mercury, cadmium, and tellurium in which the mercury to cadmium stoichiometry ratio can be adjusted to the infrared frequency. Such materials are known to those skilled in the art as HgCdTe (mercury cadmium telluride), which encompasses a broad spectrum. Ternary alloys such as HgCdTe materials are currently attracting attention as materials more suitable for secondary generation infrared detection systems.

Despite the wide acceptance of the use of prior art mercury-cadmium-tellurium compound materials, these materials suffer from various problems.
This problem is due to the size and purity of the crystals, the finish to integrated circuit technology, and the material surface due to anodization, which creates an interface that cannot withstand photovoltaic currents even in 100°C environmental conditions. relative instability, etc. Therefore, the conventional technology HgCdTe
In addition to improving the technology of using materials,
It would be desirable to find materials suitable for use in infrared radiation detection devices that also have advantages over those of existing systems.

<Means for Solving the Problems> According to the invention, an infrared radiation detection device is provided which basically uses a semiconductor heterojunction (or heterobarrier) structure, preferably of materials of the - group, especially using gallium arsenide and aluminum gallium arsenide. provided. To create two heterojunctions, two types of (AlGa)As with different component ratios are used. The heterojunction closer to the substrate is designed to have a conductive band energy barrier that is less than the amount of energy carried by infrared photons. Electrons are photoexcited, cross this coupling barrier, and are collected at the second heterojunction. The second heterojunction is designed to have a significantly larger conductive charge energy barrier than the previous heterojunction closer to the substrate. The charges collected at the second heterojunction are read out as an image signal.
The energy change in the conduction band becomes discontinuous due to the negative voltage spreading across the interface of the second heterojunction. This charge naturally occurs because excess electrons are trapped in the second heterojunction.

Although desired results can be obtained with the above structure, the above infrared imaging device has a problem of poor quantum efficiency. The reason for this is that a method is used in which almost free carriers (electrons) are excited, and this method cannot satisfy the law of conservation of energy and momentum. A photon, a massless particle, has very little momentum compared to its energy. When a photon is completely absorbed and all of the photon's energy is converted into electron momentum, the electron has a very large amount of momentum. In crystals, this generally requires the participation of phonons (acoustons) in the light absorption process. This means that the cross-sectional area for absorbing photons becomes very small.

In order to increase quantum efficiency in the process of directly absorbing photons, an artificial superlattice structure material is used instead of gallium arsenide in the infrared radiation absorption layer. In this superlattice structure, since it is a regular (lattice) structure, the conduction bands in successive energy bands are separated by very small band gaps. The lattice structure preferably includes N-type doped gallium arsenide. This gallium arsenide contains aluminum instead of gallium at regular intervals. The spacing at which the aluminum is placed is determined by the frequency of the infrared radiation being detected.

Therefore, the present invention discloses a junctionless infrared radiation detection device. In this detection device, photons are absorbed using conduction band transitions within the absorption layer. Carriers excited by conduction band transitions are injected across the injection barrier. This injection barrier consists of a heterojunction. This carrier is trapped and collected by a second heterojunction carrier collection barrier. The carriers collected by the carrier collection barrier are further periodically sensed. Preferably, the absorption layer is a superlattice structure, in which infrared photons do not have to be absorbed by indirect conduction band transitions as in normal semiconductors, but can be absorbed by essentially direct conduction band transitions. .

The infrared radiation detection device according to the invention has a conductive material substrate of a -group material, preferably gallium arsenide. On this substrate, the above-mentioned superlattice structure is formed in which thin layers of large energy gap and small energy gap semiconductor materials are stacked on top of each other. Preferred semiconductor materials are aluminum gallium arsenide and gallium arsenide. A semiconductor material alloy layer with a slight band gap is formed over this lattice structure. The semiconductor material is preferably aluminum gallium arsenide, into which photon-excited carriers are injected. A second semiconductor material alloy layer having a larger bandgap than the semiconductor alloy layer is formed over the semiconductor layer. This second semiconductor material alloy layer is
It is preferably made of aluminum gallium arsenide having a different gallium/arsenic component ratio than the semiconductor material alloy layer.

The first 2 produced by the regularity of the superlattice structure
The superlattice structure is designed such that the energy gap between the two "small bands" is equal to or less than the infrared photon energy. Doping is performed so that the first "small band" is substantially filled with free carriers. The infrared radiation is therefore efficiently absorbed, generating free carriers in the second "small band" and so on. the second semiconductor material such that such optically excited carriers cross the heterojunction closer to the substrate and reach the semiconductor material layer, but carriers in the smaller energy bands of the smaller bands are blocked; The bandgap of the layer is selected. When an electric field is applied, these carriers are allowed to migrate across the layer and are collected at the heterojunction or interface of the second semiconductor material layer where the bandgap is large. Optionally, a layer of insulator is provided on the top surface of the device, over which a metal gate electrode layer is formed.

Thus, according to this invention, HgCdTe
The effects obtained with ternary alloys are easier to handle.
It can be realized with GaAs, GaAlAs, etc. In other words, GaAs and GaAlAs have advanced process knowledge, are themselves inexpensive and stable, and are suitable for IC implementation, making it possible to create effective infrared detection devices.

(Example) Referring to FIG. 1, a substrate 11 of gallium arsenide
A typical infrared light imaging device is shown having a plurality of infrared radiation detection cells 1 formed thereon. As shown in the cross-sectional view of FIG. 2, this substrate 11 is either completely undoped or slightly doped. Each cell 1 is wired in a row and column matrix and is connected to a column line 5 and a row line 7 via a FET device 3. With such wiring, the configuration is such that any cell 1 can be called out by a well-known method.

As shown in FIG. 2, each cell 1 preferably comprises a conductive gallium arsenide substrate 11 (e.g. 10 ¹⁷
It is doped to N type with an impurity concentration of . )
have. Covering this substrate 11, an N-type gallium arsenide layer 13 having a superlattice structure is formed. Between the substrate 11 and the layer of semiconductor material 15, aluminum is included at various levels in place of some gallium. The interface between the photon absorption layer 13 and the photon collection layer 15 is provided by aluminum discontinuities. At this interface, a predetermined amount of aluminum, which is larger than other parts, is included in place of some of the gallium, forming a heterojunction 17 between gallium arsenide and aluminum gallium arsenide. The heterojunction 17 limits the remaining electrons in the absorption layer 13 of the lattice structure. Barrier layer 19 preferably comprises a second aluminum gallium arsenide layer 1 containing aluminum in a greater proportion than layer 15 (having a larger bandgap).
9 and a second layer between layer 15 and layer 19.
The heterojunction 21 is constructed such that the amount of aluminum increases rapidly. By suddenly increasing the amount of aluminum in layer 19 at the interface,
Since the interface has a large band gap, it is optically excited and crosses the heterojunction 17 to form the layer 15.
The incoming electrons are blocked by the barrier formed by the heterojunction 21 and provide an electron collection point for collecting the electrons. selectively layer 19
Insulator layer 23 can be formed instead of or over layer 19. The insulating material for this insulating layer 23 is preferably silicon dioxide. Preferably, a transparent conductive material layer 25 is formed covering this insulating material layer 23. As transparent conductive materials, tin oxide and indium tin oxide (ITO) are suitable for this purpose.

Referring to FIG. 3, layer 13 and layer 13
The superlattice structure of is shown enlarged. As is clear from the figure, N-type doped gallium arsenide 31 is deposited on a substrate 11 with a predetermined layer thickness by molecular beam epitaxy (MBE).
formed on top. Aluminum is added to a portion of this gallium arsenide layer 31 to form a thin aluminum gallium arsenide layer 33. This process is repeated alternately throughout the layer 13 until it reaches a predetermined height. The regions 33 containing aluminum are stacked on top of each other at regular intervals, the spacing between these layers being determined by the wavelength of the infrared radiation received. It can thus be seen that the configuration of layer 13 is adjusted to the frequency of the infrared radiation to be detected. With the structure of FIG. 3, the quantum efficiency of the infrared radiation detection element is greatly improved.

In actual operation, photons of infrared radiation impinge on layer 13 and are converted into electron energy, thereby exciting electrons in layer 13. Electrons in these layers 13 that have received sufficient energy from the photons are now able to climb the barrier or heterojunction 17 that exists between layers 13 and 15, as shown in FIG. These electrons therefore migrate over the barrier 17 into the layer 15. layer 1
The electrons that have moved into layer 5 are transferred between layer 15 and layer 19 (or, if layer 19 is not used, layer 15 and layer 2).
3) and is collected here without being able to cross the barrier or heterojunction 21 between them. Since the electrons are collected at the barrier 21, when the cell is properly sensed using the row lines 5 and column lines 7 as shown in FIG. It can be seen that the charge of each formed capacitor to be sensed can be detected.

The operation of various embodiments of the apparatus of the invention will now be shown in more detail.

FIG. 5 shows the electronic structure of the conduction band of the superlattice material layer 13 in more detail. The regularity of the superlattice structure creates an artificial Brillouin band boundary 101. This boundary 101 deforms the normal parabolic potential curve 103 and slightly deforms it near the artificial Brillouin band boundary 101 instead of such a normal parabolic potential curve 103, creating a first small band 105. Create. Furthermore, the upper part of the parabolic potential curve 103 is also deformed and moved by the periodicity of the spatial arrangement of atoms in the superstructural structure,
A second small band 107 is provided. An advantage of such a structure is that it is possible to obtain a transition directly within the conduction band. That is, the transition from the first small band to the second small band can be obtained with a wavelength K having a constant value.
This means that the absorption of photons does not require the help of acoustic molecules (phonons). This means that photons are absorbed very efficiently, as explained above. The height of the injection barrier must be chosen to be lower than the height of the energy gap of the second small band.
Thereby, carriers injected into the second small band by absorption of photons are injected over the injection barrier, as explained above.

This structure allows very sharp cut-off of long wavelengths. If the absorption band of the structure is too broad, it is of course simply possible to cover the surface of the structure with a material that absorbs shorter wavelengths.

Preferably, an N-type background material is used because the superlattice structure takes advantage of the fact that the mass of electrons is smaller. Superlattice materials consist of very thin layers of different materials that are stacked one on top of the other in a periodic manner.
It is constructed to have periodic barriers. The layers of different materials are preferably thin enough (about a monolayer in this example) to allow the carrier to tunnel between the layers. This means that carriers generated in any part of the superlattice structure 13 can pass through the superlattice structure 13, overcome the barrier, and be injected into the collection layer 15. The alternating thin layers can be composed of semiconductor layers with different bandgaps. However, it can be constructed from almost any material as long as it has a structure that perfectly maintains the crystal lattice structure while periodically disturbing the characteristics of the lattice structure. It is.

Thus, as is known in the art, superlattice structures can be formed in configurations that include thin metal or insulator layers with intervening layers of semiconductor material.

A preferred embodiment of the present invention combines the carrier injected into collection layer 15 and barrier layer 19 into collection layer 15 and barrier layer 19.
Although a photocapacitive element structure is used which collects at a heterojunction 21 or a barrier for charge collection between the two, this is not necessary. Second aspect of the present invention
In the embodiment, the carrier injected into the collection layer 15 is configured as a photoconductor, where it is not only stored in the device by means of a collection barrier, but is also utilized as an output current. Furthermore, other embodiments are also possible. For example, the carriers injected into the collection layer 15 can be configured to be collected by MIM (metal-insulator-metal) capacitive elements or in other ways. An important feature of the invention, which is indispensable in any case, is that the superlattice structure is arranged adjacent to a heterojunction having a barrier of a predetermined height, the height of the barrier of this heterojunction being equal to the height of the superlattice structure. The superlattice structure is designed to have an energy equal to the intermediate energy between the energies of the first and second small bands, so that carriers causing a transition to the second small band of the superlattice structure are injected over the injection heterojunction barrier. and thereby produce an output signal corresponding to a photon of infrared radiation.

It should be noted that the technology for -group compound semiconductor materials is so advanced and well understood that embodiments of the present invention have been described herein primarily with respect to these materials. That is, it is easy to form epitaxial layers of gallium arsenide and aluminum gallium arsenide by molecular beam epitaxy (MBE). A particular advantage of systems constructed from such materials is that the lattice of the gallium arsenide and aluminum gallium arsenide layers is very well matched and the interface between these layers is highly stable. Epitaxial heterojunctions between materials of the - family with different band gaps can also be used with other materials well known in the art. Regarding such materials, see, for example, ``Optical Processing of Semiconductors'' by Pankove, published in 1971. This book is incorporated herein by reference.

Furthermore, while molecular beam epitaxy is of course the preferred method for forming this structure, it is not the only method. Researchers working in the field of chemical vapor deposition methods, such as organometallic chemical vapor deposition, have reported good results with step heteroepitaxy and the creation of thin layers with this technique. However, molecular beam epitaxy is currently a reliable and suitable manufacturing technique. In particular, using molecular beam epitaxy, a superlattice material layer having a thickness approximately equivalent to a monolayer can be produced very easily.

In the preferred embodiment shown above, preferably:
N-type gallium arsenide is used for the thick layer included in the superlattice structure. The regularity of the superlattice structure is calculated according to the energy of the photon with the longest wavelength among the photons to be detected. This formula is shown below. That is,
The spacing L of the superlattice structure is chosen such that h ² ×K ² /2m ^* = photon energy, where K = 2π/L.

Therefore, detecting photons with long wavelengths requires a superlattice structure with a longer period. For radiation at a wavelength of 5 microns, the energy of the photon (h x ν) is approximately
It is 0.25 electron volts or 4 x 10 ^-20 joules. In gallium arsenide, the effective mass of the electron m ^*
is equal to about 7% of the isolated mass m ₀ and h represents Planck's constant, so from this formula, the superlattice spacing is
It is shown that it is 93 Å.

Doping to make gallium arsenide N-type is
Preferably, it is carried out at a rather high impurity concentration to generate a large number of carriers and thus provide a suitable cross section for the absorption of photons. In a preferred embodiment, the absorbent layer 1
The doping level of 3 is high, preferably 2×
It can be done with impurity concentrations of 10 ¹⁸ or higher, providing a large cross section for photon absorption.
Preferably, the collection layer 15 is composed of a substantially intrinsic semiconductor material, ie a material that is not intentionally doped.

Two photocapacitive element embodiments have been described. In a first embodiment, the collection layer forms a heterojunction at the interface with an epitaxial barrier layer with a larger bandgap, such as intrinsic aluminum arsenide, above which the gate 25
is formed. In the second embodiment, the signal charge is
Collected within the MIS diode. This diode has, for example, an insulator layer 23 and a conductive material layer 23 on the collector layer 15 .

The maximum impurity concentration that can actually be implanted into the absorption layer 13 is determined by the dark current. That is, the number of carriers in the first small band must not be too large so that the carriers do not end up in the second small band.

When the carrier enters the second small band, dark current flows in the collection layer. In addition, in order to avoid providing a large number of carriers within a width of several KT from the injection barrier height (top) at the operating temperature, that is, to prevent the generation of excessive dark current, the first small band is The number of carriers within should not be too large.

In general, the absorption layer 13 can be further modified in order to reduce the polarization sensitivity and thereby improve the ratio of signal current to dark current. The structure of this layer is modified to have not only regularity in the vertical direction as explained above, but also regularity in the horizontal direction with dimensions corresponding to the regularity in the vertical direction. Such a structure with regularity in the horizontal direction improves the quantum efficiency of the device, but inevitably increases the production cost.
Therefore, it is not very desirable at present.

The present invention is suitable for long wavelength light absorption.
Please note that we have suggested the use primarily of materials from the family. Although the - group superlattice structure was mentioned earlier, - group materials and the processing steps for the properties of these materials are generally better known and easier to work with than - group materials.

<Advantages> The device of the present invention described above clearly has the advantage that the effective area of the substrate is expanded, more advanced integrated circuit device technology can be constructed, and it has an interface with improved stability suitable for storing charge. It can be seen that this device is superior to the conventional mercury-cadmium-tellurium infrared radiation detection device. Furthermore, in the device of the present invention,
Only a very small dark current is generated compared to the photocurrent.

Although the invention has been particularly described with respect to specific embodiments thereof, various modifications and changes will become apparent to those skilled in the art.
It is therefore intended that the present invention be construed in the broadest manner in the context of the prior art and in accordance with the scope of the appended claims, including all such modifications.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of an imaging device using a photocapacitive detector according to the present invention, FIG. 2 is a schematic diagram of a photocapacitive detector according to the present invention, and FIG. 3 is a superstructural structural material of FIG. 2. An enlarged view of layer 13, FIG. 4 shows the conduction band distribution of FIGS. 2 and 3, and FIG. 5 shows the superlattice to show the first and second small bands formed within the conduction band. FIG. 3 is a diagram showing the band structure of the structure in more detail; 1... Infrared radiation detection cell, 3... FET device,
5... Column line, 7... Row line, 11... Gallium arsenide substrate, 13... Absorption layer, 15... Collection layer, 17...
...first heterojunction, 19...barrier layer, 21...
...Second heterojunction, 23...Insulator layer, 25...
...transparent electrode layer, 31...N-type doped gallium arsenide layer, 33...aluminum gallium arsenide layer,
101... Brillouin band boundary, 103... Potential curve, 105... First small band, 107... Second small band.

Claims (1)

Claims: 1. (a) having an absorbing superlattice of at least two alternating bands of first and second type;
(b) has a photon collection layer adjacent to said superlattice, which photon collection layer is smaller than said second type and has a wider bandgap than said first type; has a large bandgap potential, and further comprises (c) a sensor for detecting electrons jumping from the superlattice to the photon collection layer by photon energy striking the absorption superlattice. Infrared semiconductor detection device.